A fuel cell is an electric cell that converts the chemical energy of a fuel, typically hydrogen, directly into electric energy in a continuous process. Although fuel cells can be used with a variety of fuels and oxidants, they almost exclusively combine hydrogen and oxygen to form water vapor. Fuel cells include an anode in contact with the fuel, a cathode in contact with the oxygen and an electrolyte sandwiched between the anode and cathode. Each cell creates less than one volt so that a series or stack of fuel cells are used to convert fuel into usable energy. Interconnect plates are used between each cell to keep the fuel and oxygen separated and to electrically connect the anode of one cell to the cathode of an adjacent cell.
One source of hydrogen is natural gas. A common way to obtain hydrogen from natural gas is by using a reformer which combines natural gas and steam at a high temperature, such as 760.degree. C., to obtain the hydrogen. Some fuel cells operate using a separate, external reformer to create the hydrogen; other fuel cells combine the function of a reformer into the fuel cell itself by operating the fuel cell at a high enough temperature, as well as other appropriate design considerations.
One type of fuel cell uses radial flow configurations for solid oxide fuel cells. In one design, disclosed in M. Petrik et al., "Stack Development Status of the Interscience Radial Flow (IRF) SOFC", An EPRI/GRI Fuel Cell Workshop on Fuel Cell Technology Research and Development, Atlanta, Ga., Mar. 22-23, 1994, the fuel and air are fed to each cell through a pair of holes at the center region of the cell. The fuel and air then flow radially outwardly to the edge of the cell. This flow configuration requires seals to segregate the fuel and air at the feed points and also runs the risk of temperature excursions at the center of the cell where both rich fuel and rich oxygen exist. In another configuration, disclosed in M. Prica et al., "Contoured PEN Plates for Improved Thermomechanical Performance in SOFCs", Proceedings of the Second European Fuel Cell Forum, Vol. 1, pp. 393-402, Oslo, Norway, May 6-10, 1996, the fuel and air are fed to the center of each cell through a pair of needles. These gases then flow radially to the cell edge. This flow configuration eliminates the gas seal requirement but still has problems with regard to temperature excursion. In another configuration, disclosed in European Patent 0,635,896 A1, the fuel is fed to the center of the cell by a feed needle while air is fed to the entire cathode area by distribution nozzles. The spent fuel and spent air are collected at the cell edge. This configuration eliminates the need for a gas seal and does not have temperature excursion problems. It does, however, require a complex gas nozzle distribution system.